The invention relates to a process for cleaning an inkjet printing head which has nozzle openings and ink channels which lead to them, in which electrical drive elements for ejecting ink out of the nozzle openings are located, in which the nozzle openings and the ink channels are forcibly flushed with cleaning liquid. A device according to this process for cleaning an inkjet printing head with a holding device for the printing head, connecting means for liquid-tight coupling of a detergent line to the nozzle openings of a printing head inserted into the holding device, and a flushing device for delivering the cleaning liquid to the detergent line is likewise the subject matter of the invention.
Designs of inkjet printers are known in which the inkjet printing heads are each nondetachably connected by an ink tank to an ink cartridge with an integrated printing head. The printer manufacturers design these ink cartridges as disposable products which are to be replaced by filled new products as soon as the ink reservoir in the ink tank has been used up.
Over time, in the nozzle openings and in the fine ink channels which lead to them deposits and contaminants can collect, for example due to dried ink residues. These contaminants can constrict or block the ink channel in the printing head or the nozzle openings so that individual nozzles no longer work properly or can fail and the print image is adversely affected. If, as intended by the printer manufacturers, a new printing head is used with each ink refill, these adverse effects are largely prevented. In any case the frequent replacement of high quality printing heads entails a considerable production expense and causes high costs which are passed on to the final consumer by the manufacturer. In addition the environment is also burdened by the discarded printing heads.
For these reasons it is a good idea to reprocess used ink cartridges for re-use. The reprocessing comprises refilling of the ink cartridge with new ink and reconditioning of the printing head which is ensured essentially by thorough removal of all contaminants. Because the possible ink throughput of the printing heads is a multiple of the charge of the ink cartridge, these refill products are generally qualitatively equivalent to the new products.
The quality of the refill products depends directly on the fact that the printing heads are cleaned to be as free of residue as possible without wearing or damaging the fine structures of the ink channels and nozzle openings. To do this, forced flushing has long been used, in which cleaning liquid is flushed under pressure from the outside through the nozzle openings, therefore against the flow direction of the ink in printing operation (“reverse flushing”). In this way it is possible to remove even sticking foreign bodies to the inside from the nozzle openings which generally run conically to the outside.
The cleaning action by simple flushing processes is however limited by the fact that the nozzle openings are fluidically connected to one another during flushing so that only a relatively low overpressure can build up in front of a constricted or blocked cross section. If the overpressure is not enough to detach and wash away the contaminant, satisfactory cleaning is not possible and the pertinent printing head is ultimately unusable.
To improve the cleaning action, EP 1 101 616 A1 has already proposed exciting the printing head from the outside with ultrasound during flushing, similarly to the known ultrasonic cleaning baths. But it has been found in practice that the nozzle plates of the printing heads, which plates are provided with nozzle openings, and to some extent the drive elements—heating elements or piezo-oscillators—are damaged. With external excitation with high energy ultrasound harmful resonance peaks can occur. In any case improvement of cleaning by exposure to sonic waves from the outside cannot be achieved. In addition the integration of an ultrasonic means into the flushing means is relatively complex.
In view of the above described problem, an objective of the invention is to devise a process which has improved cleaning action compared to simple forced flushing, however is more gentle and less complex than the known ultrasonic cleaning processes. A device for carrying out the process will likewise be devised.